Nuclear power stations having one or more reactors in which fuel assemblies are used, generally include one or more fuel buildings adjacent to the reactor building and in which there are performed operations of removing spent fuel assemblies, of examining and repairing said fuel assemblies, and of preparing to load the reactor with new assemblies or with refurbished assemblies. The fuel building generally contains a spent fuel storage pool which can be put into communication with a pool of the nuclear reactor. Within the fuel pool, it is generally necessary to perform complex operations of examining and measuring spent fuel assemblies and possibly also operations of repairing and refurbishing fuel assemblies.
Fuel assemblies for water-cooled nuclear reactors, and in particular fuel assemblies for nuclear reactors cooled by pressurized water, which are in the form of right prisms and are disposed vertically in the core of the reactor, are of great length and small cross-section, where these assemblies can present a length of about 4 meters (m) or even more and a square section having a side of about 0.20 m. Fuel rods are inserted into the fuel assembly support structure to constitute a bundle occupying the major part of the length of the fuel assembly.
In order to perform the operations of inspecting, visually examining, or taking measurements on fuel assemblies, it is necessary to have a tall structure (e.g. about 5 m tall) into which each of the fuel assemblies for inspection is inserted in succession in a vertical disposition, the support structure also having means for guiding and moving auxiliary devices for examining, inspecting, or taking measurements, in particular for moving the devices in a vertical direction.
The installation for inspecting fuel assemblies constitutes a structure that is tall and slender, and that needs to be placed inside the fuel pool so as to stand on the bottom of the pool in a zone where fuel assemblies are to be inspected and examined.
It is generally advantageous to make installations for inspecting and examining fuel assemblies as to enable them to be transported from one fuel building to another, on a given site or even between different nuclear power station sites. This makes the use of such installations more profitable, given that they are needed only from time to time in the lifetime of a nuclear power station, e.g. when loading or reloading fuel assemblies into the core of a nuclear reactor.
It is also desirable for it to be possible to place the examination and inspection installation in a zone of the fuel pool that can be selected as a function of the position of the spent fuel assembly storage racks in the fuel pool. It is therefore necessary to have structure support means that are entirely stable, enabling the vertical position of the structure to be adjusted, and including means for resting on the bottom of the pool that can easily be separated from the bottom of the pool or put into place and adjusted in order to install the structure in a pool.
The bottom of a fuel pool is generally constituted by a slab of concrete covered by a covering of stainless steel which constitutes the support surface of the structure. Said support structure may present certain departures from planeness and/or from the horizontal, so it must be possible to adjust the vertical direction of the structure supporting the examination and inspection installation before it is put into operation, since it is necessary for the installation to be vertical in order for the auxiliary examination and inspection means to be moved and adjusted in satisfactory manner.
In addition, it can be necessary to provide antiseismic support for the examination and inspection installation structure so as to avoid any risk of the inspection structure toppling or being destroyed, and to avoid any risk of the fuel assembly under examination being damaged, in the event of an earthquake of potentially large magnitude occurring, as can happen in at least some zones where nuclear power stations are installed.
Various types of support device or antiseismic pad are known that can be used for protecting certain buildings, in particular dwellings or industrial buildings such as the buildings of a nuclear power station, or indeed civil engineering works such as bridges. Such antiseismic support devices comprise a support base that is permanently secured to a support surface, which surface may be the surface of the ground, a foundation, or a support element of a civil engineering work, a support and bearing plate secured to the structure that is to be supported, or sliding or rolling means interposed between the support plate and the support base.
For example, as described in U.S. Pat. No. 5,599,106 and in WO 01/42593, it is possible to use rolling elements such as balls or rollers interposed between bearing plates having rolling and bearing surfaces for the rolling elements. A bottom bearing plate of the support device is permanently secured to the ground or to a civil engineering work. At least one top bearing plate resting on the rolling elements is designed to move in limited manner relative to the support base, in the event of an earthquake leading to forces being applied in arbitrary directions.
Such devices which are anchored to the ground or to a civil engineering element and which remain permanently on the ground or on the civil engineering element, cannot be used in support pads for a transportable structure that needs to be put into place preferably simultaneously with its antiseismic support pads and that must include adjustment means, in particular for adjusting its vertical direction.
Rolling means are also known that are referred to as handling balls which enable heavy objects to be moved in any direction on a bearing surface or against a guide surface. Such handling balls comprise a spherical rolling element mounted to rotate freely in a ball bearing secured to a support base for the handling ball, for supporting it on a support or guide surface. Such handling balls with very low friction and having a center of spherical rotation permitting displacements in arbitrary directions can be most advantageous in antiseismic support devices. Until now such devices have been restricted to the field of handling.